Reduced internal stress of quasi-single crystalline Na4Fe3(PO4)2P2O7 electrode enhancing sodium-ion kinetics

A NASICON-type quasi-single crystalline Na4Fe3(PO4)2P2O7 (SC) cathode material was successfully synthesized via a freeze-drying method. The as-optimized SC sample displays both outstanding rate performance (107.9 and 88.5 mAhg−1 at 0.1C and 5C, respectively) and excellent cycling stability (88.1 % of capacity retention after 1000 cycles at 1C). Reduced sodium ion migration path of as-prepared SC cathode material effectively enhanced the kinetics of Na+ upon cycling. More importantly, the SC structure could suppress nearly 50 % of internal strain, prodigiously reducing the local stress accumulation accompanied by the inhomogeneous sodium concentration gradient during the charging/discharging processes. The smaller the stress accumulation, the more stable the crystal structure would be obtained, which maintained a more superb mechanical stability of the SC cathode material. At the same time, a high-spin state of Fe in the SC sample was confirmed by both of electron paramagnetic resonance and temperature-dependent magnetization susceptibility, indicating that the eg orbital occupation of Fe2+ could be regulated to optimize the bond strength between the reduction and oxidation processes. As a consequence, the band gap of SC material has decreased from 1.66 to 1.45 eV, and the electronic conductivity has increased from 6.0 to 32.4 μS/cm. It is believed that the SC cathode material could be a competitive candidate material for sodium ion batteries.